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Existence of Periodic Solutions for a Delayed RatioDependent ThreeSpecies PredatorPrey Diffusion System on Time Scales
Advances in Difference Equations volume 2009, Article number: 141589 (2009)
Abstract
This paper investigates the existence of periodic solutions of a ratiodependent predatorprey diffusion system with MichaelisMenten functional responses and time delays in a twopatch environment on time scales. By using a continuation theorem based on coincidence degree theory, we obtain suffcient criteria for the existence of periodic solutions for the system. Moreover, when the time scale is chosen as or , the existence of the periodic solutions of the corresponding continuous and discrete models follows. Therefore, the methods are unified to provide the existence of the desired solutions for the continuous differential equations and discrete difference equations.
1. Introduction
The traditional predatorprey model has received great attention from both theoretical and mathematical biologists and has been studied extensively (e.g., see [1–4] and references therein). Based on growing biological and physiological evidences, some biologists have argued that in many situations, especially when predators have to search for food (and therefore, have to share or compete for food), the functional response in a preypredator model should be ratiodependent, which can be roughly stated as that the per capita predator growth rate should be a function of the ratio of prey to predator abundance. Starting from this argument and the traditional preydependentonly mode, Arditi and Ginzburg [5] first proposed the following ratiodependent predatorprey model:
which incorporates mutual interference by predators, where is a MichaelisMenten type functional response function. Equation (1.1) has been studied by many authors and seen great progress (e.g., see [6–11]).
Xu and Chen [11] studied a delayed twopredatoroneprey model in two patches which is described by the following differential equations:
In view of periodicity of the actual environment, Huo and Li [12] investigated a more general delayed ratiodependent predatorprey model with periodic coefficients of the form
In order to consider periodic variations of the environment and the density regulation of the predators though taking into account delay effect and diffusion between patches, more realistic and interesting models of population interactions should take into account comprehensively other than one or two aspects. On the other hand, in order to unify the study of differential and difference equations, people have done a lot of research about dynamic equations on time scales. The principle aim of this paper is to systematically unify the existence of periodic solutions for a delayed ratiodependent predatorprey system with functional response and diffusion modeled by ordinary differential equations and their discrete analogues in form of difference equations and to extend these results to more general time scales. The approach is based on Gaines and Mawhin's continuation theorem of coincidence degree theory, which has been widely applied to deal with the existence of periodic solutions of differential equations and difference equations.
Therefore, it is interesting and important to study the following model on time scales :
with the initial conditions
where . In (1.4), represents the prey population in the th patch , and represents the predator population. is the prey for , and is the prey for so that they form a food chain. denotes the dispersal rate of the prey in the th patch . For the sake of generality and convenience, we always make the following fundamental assumptions for system (1.4):

(H)
are all rdcontinuous positive periodic functions with period ; are nonnegative constants.
In (1.4), set . If , then (1.4) reduces to the ratiodependent predatorprey diffusive system of three species with time delays governed by the ordinary differential equations
If , then (1.4) is reformulated as
which is the discrete time ratiodependent predatorprey diffusive system of three species with time delays and is also a discrete analogue of (1.6).
2. Preliminaries
A time scale is an arbitrary nonempty closed subset of the real numbers . Throughout the paper, we assume the time scale is unbounded above and below, such as and . The following definitions and lemmas can be found in [13].
Definition 2.1.
The forward jump operator , the backward jump operator , and the graininess are defined, respectively, by
If , then is called rightdense (otherwise: rightscattered), and if , then is called leftdense (otherwise: leftscattered).
If has a leftscattered maximum , then ; otherwise . If has a rightscattered minimum , then ; otherwise .
Definition 2.2.
Assume is a function and let . Then one defines to be the number (provided it exists) with the property that given any , there is a neighborhood of such that
In this case, is called the delta (or Hilger) derivative of at . Moreover, is said to be delta or Hilger differentiable on if exists for all . A function is called an antiderivative of provided for all . Then one defines
Definition 2.3.
A function is said to be rdcontinuous if it is continuous at rightdense points in and its leftsided limits exists (finite) at leftdense points in . The set of rdcontinuous functions will be denoted by .
Definition 2.4.
If , , and is rdcontinuous on , then one defines the improper integral by
provided this limit exists, and one says that the improper integral converges in this case.
Definition 2.5 (see [14]).
One says that a time scale is periodic if there exists such that if , then . For , the smallest positive is called the period of the time scale.
Definition 2.6 (see [14]).
Let be a periodic time scale with period . One says that the function is periodic with period if there exists a natural number such that , for all and is the smallest number such that .
If , one says that is periodic with period if is the smallest positive number such that for all .
Lemma 2.7.
Every rdcontinuous function has an antiderivative.
Lemma 2.8.
Every continuous function is rdcontinuous.
Lemma 2.9.
If and , then

(a)
;

(b)
if for all , then ;

(c)
if on , then .
Lemma 2.10.
If , then is nondecreasing.
Notation.
To facilitate the discussion below, we now introduce some notation to be used throughout this paper. Let be periodic, that is, implies ,
where is an periodic function, that is, for all , .
Notation.
Let be two Banach spaces, let be a linear mapping, and let be a continuous mapping. If is a Fredholm mapping of index zero and there exist continuous projectors and such that , , then the restriction is invertible. Denote the inverse of that map by . If is an open bounded subset of , the mapping will be called compact on if is bounded and is compact. Since is isomorphic to , there exists an isomorphism .
Lemma 2.11 (Continuation theorem [15]).
Let be two Banach spaces, and let be a Fredholm mapping of index zero. Assume that is compact on with is open bounded in . Furthermore assume the following:

(a)
for each ;

(b)
for each ;

(c)
.
Then the operator equation has at least one solution in .
Lemma 2.12 (see [16]).
Let . If is periodic, then
3. Existence of Periodic Solutions
The fundamental theorem in this paper is stated as follows about the existence of an periodic solution.
Theorem 3.1.
Suppose that (H) holds. Furthermore assume the following:
 (i)

(ii)
,

(iii)
,

(iv)
,
then the system (1.4) has at least one periodic solution.
Proof.
Consider vector equation
Define
where is the Euclidean norm. Then and are both Banach spaces with the above norm . Let . Then
and . Since is closed in , then is a Fredholm mapping of index zero. It is easy to show that are continuous projectors such that . Furthermore, the generalized inverse (to ) exists and is given by , thus
Obviously, are continuous. Since is a Banach space, using the ArzelaAscoli theorem, it is easy to show that is compact for any open bounded set . Moreover, is bounded, thus, is compact on for any open bounded set . Corresponding to the operator equation , we have
Suppose that is a solution of (3.5) for certain . Integrating on both sides of (3.5) from to with respect to , we have
It follows from (3.5) to (3.9) that
Multiplying (3.6) by and integrating over gives
which yields
By using the inequality , we have
Then
By using the inequality , we derive from (3.17) that
Similarly, multiplying (3.7) by and integrating over , then synthesize the above, we obtain
It follows from (3.18) and (3.19) that
so, there exists a positive constant such that
which together with (3.19), there also exists a positive constant such that
This, together with (3.11), (3.12), and (3.21), leads to
Since , there exist some points , such that
It follows from (3.21) and (3.22) that
From (3.8) and (3.9), we obtain that
This, together with (3.12), (3.13), and (3.26), deduces
From (3.6) and (3.24), we have
From (3.7) and (3.24), it yields that
Noticing that , from (3.8) and (3.9), deduces
There exist two points such that
Hence,
where . Then, this, together with (3.12), (3.13), (3.23), (3.28), (3.29), and (3.32), deduces
It follows from (3.27) to (3.33) that
From (3.34), we clearly know that are independent of , and from the representation of , it is easy to know that there exist points such that , where
Take , where is taken sufficiently large such that , and such that each solution of the system satisfies if the system (3.35) has solutions. Now take . Then it is clear that verifies the requirement (a) of Lemma 2.11.
When , is a constant vector in with , from the definition of , we can naturally derive whether the system (3.35) has solutions or not. This shows that the condition (b) of Lemma 2.11 is satisfied.
Finally, we will prove that the condition (c) of Lemma 2.11 is valid. Define the homotopy by
where
where is a parameter. From (3.37), it is easy to show that . Moreover, one can easily show that the algebraic equation
has a unique positive solution in . Note that (identical mapping), since , according to the invariance property of homotopy, direct calculation produces
where is the Brouwer degree. By now we have proved that verifies all requirements of Lemma 2.11. Therefore, (1.4) has at least one periodic solution in . The proof is complete.
Corollary 3.2.
If the conditions in Theorem 3.1 hold, then both the corresponding continuous model (1.6) and the discrete model (1.7) have at least one periodic solution.
Remark 3.3.
If and in (1.6), then the system (1.6) reduces to the continuous ratiodependence predatorprey diffusive system proposed in [17].
Remark 3.4.
If we only consider the prey population in onepatch environment and ignore the dispersal process in the system (1.4), then the classical ratiodependence two species predatorprey model in particular of (1.4) with MichaelisMenten functional response and time delay on time scales
where are positive periodic functions, is nonnegative constant. It is easy to obtain the corresponding conclusions on time scales for the system (3.40).
Corollary 3.5.
Suppose that (i) , (ii) hold, then (3.40) has at least one periodic solution.
Remark 3.6.
The result in Corollary 3.5 is same as those for the corresponding continuous and discrete systems.
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Acknowledgments
The author is very grateful to his supervisor Prof. M. Fan and the anonymous referees for their many valuable comments and suggestions which greatly improved the presentation of this paper. This work is supported by the Foundation for subjects development of Harbin University (no. HXK200716) and by the Foundation for Scientific Research Projects of Education Department of Heilongjiang Province of China (no. 11513043).
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Keywords
 Periodic Solution
 Functional Response
 Jump Operator
 Continuation Theorem
 Fredholm Mapping